Wireless communication has extensive applications in consumer and business markets. Among the many communication applications/systems are: fixed wireless, unlicensed Federal Communications Commission (FCC) wireless, local area network (LAN), cordless telephony, personal base station, telemetry, mobile wireless, and other digital spread spectrum communication applications. While each of these applications utilizes spread spectrum communications, they generally utilize unique and incompatible modulation and protocols. Consequently, each application may require unique hardware, software, and methodologies for demodulation. This practice can be costly in terms of design, testing, manufacturing, and infrastructure resources. As a result, a need arises to overcome the limitations associated with the varied hardware, software, and methodology of demodulating digital signals in each of the varied spread spectrum applications.
A demodulator component is used in a wireless communication device for code demodulation and data demodulation of a received signal in order to provide the data signal. However, the received data signal may have impairments due to transmission and propagation delay factors. This impairment can be characterized as multipath fading in which each path exhibits a random, complex, and time-varying phase delay of the signal. Consequently, a need arises for a receiver to correct the phase error in a received signal.
Pilot signals are used in transmission protocols to help the receiver estimate an unknown channel. Essentially, a pilot signal supports estimation of an unknown random variable with known data. Coherent demodulation solves part of the phase error problem by utilizing a pilot signal having known data, e.g., a pseudonoise (PN) data sequence. The PN data sequence is known to both the transmitter and the receiver. If the transmitter sends out a known pilot signal with a known PN sequence, then the receiver can determine the phase correction using an internally generated PN sequence that is identical to that of the transmitter. To correct the phase error, a feedback loop can be provided to a radio frequency/intermediate frequency (RF/IF) transceiver to make phase adjustments. However, this requires the feedback signal to be in an analog format. Furthermore, the RF/IF transceiver is an analog device utilizing analog components, such as a voltage-controlled oscillators (VCOs) to generate a frequency. These analog components have well-known weaknesses such as temperature sensitivity, drift, etc. Thus, a need arises for a method and apparatus to overcome the limitations in conventional analog phase correction system.
Because of the nature of a feedback system, a lag in the correction of a signal occurs. That is, the received signal that has passed through demodulation prior to the correction does not receive the benefit of the corrected phase in the RF/IF transceiver. Thus, errors can be propagated through a communication device due to the intermittent phase error and the lag in the feedback correction system. This error propagation can impair the quality of service achieved using a communication device. Poor quality of service can have detrimental effects, particularly when users demand increasingly stringent performance standards. Furthermore, a feedback system to an analog device can be complicated and costly. The nature of a feedback system is a closed-loop phase tracking process. Unfortunately, a closed feedback system of this nature is not robust, especially in fading channels. As a result, a need arises for a phase correction system that overcomes some of the major limitations of a conventional feedback system.